Method for production of a very thin layer with thinning by means of induced self-support

ABSTRACT

The invention relates to a process for obtaining a thin layer made of a first material on a substrate made of a second material called the final substrate, including the following steps:
         bonding a thick layer of a first material on one of its main faces on the final substrate at an interface,   implantation of gaseous species in the thick layer of first material to create a weakened zone delimiting said thin layer between the interface and the weakened zone,   deposit a layer of third material called the self-supporting layer on the thick layer made of first material,   fracture within the structure composed of the final substrate, the thick layer of first material and the layer of third material, at the weakened zone to supply the substrate supporting said thin layer.

TECHNICAL DOMAIN

This invention relates to a process for obtaining a thin layer on asubstrate, particularly for obtaining a very thin layer, typically lessthan 0.1 μm.

It is particularly applicable to the production of an SOI typestructure.

STATE OF PRIOR ART

Document FR-A-2 681 472 (corresponding to American U.S. Pat. No.5,374,564) divulges a process for obtaining a thin silicon layer on asupport to supply an SOI type substrate. The process includes a firststep consisting of implanting a silicon substrate or an initialsubstrate by ions, for example hydrogen ions, to obtain a weakened zonedelimiting a thin layer of silicon from the substrate implantation face.During a second step, a stiffener or final substrate is bonded on theimplanted face of the initial substrate. The third step consists ofseparating the resulting stacked structure at the weakened zone.Separation produces a thin silicon layer transferred on a support, theremainder of the initial substrate being reusable. This process is knownparticularly under the name Smart Cut®.

This process is used to make a stacked structure by bonding, for exampleby molecular bonding, supporting a monocrystalline or polycrystallinethin layer. It gives very good results to obtain transfers of layers asthin as 0.1 μm. However, problems can arise when trying to obtain verythin layers (typically less than 0.1 μm) due to the appearance ofdefects, for example blisters, starting from the bonding interface.

One solution for obtaining very thin layers is to firstly obtain athicker thin layer and then to remove the surplus material until therequired thickness is obtained. However, excessive removal usingconventional techniques (chemical mechanical polishing CMP, heattreatment, chemical etching, ionic etching, etc.), reduces thehomogeneity of the thin layer. This degradation is more marked when thethickness to be removed is greater. Therefore quality, measured in termsof homogeneity of the thickness of the transferred layer, is degradedcompared with what can be obtained using the Smart Cut® process.

Another problem occurs when the materials from which the layers to bethinned have properties that make CMP thinning difficult. This is thecase for example for excessively hard materials such as sapphire, SiC,diamond. This is also the case for structures in which bonding used forstacking makes it impossible to use such techniques. For example, CMPand wet chemical etchings are unusable when the bonding energy is toolow.

The pure exfoliation method, for example generated by implantation andby heat treatment at high temperature and without stiffener (approachdescribed in American U.S. Pat. No. 6,103,599) can leave a roughnessthat is too great to be recoverable by CMP, hydrogen annealing or anyother known surface treatment. Thus, the burst blisters phenomenon(exfoliation) can leave morphologies that are very difficult to removeon the surface. These burst blisters can be compared with sequences ofsteps at low frequencies (typical widths of the order of a few tens ofμm).

PRESENTATION OF THE INVENTION

It is proposed to overcome this problem by using a process in which arelatively thick layer of material to be transferred is transferred ontothe required support, and it is then thinned by implantation andassisted fracture due to the presence of an additional layer fixed tothis thick layer. The result is a very thin good quality layer on saidsupport.

Therefore, the purpose of the invention is a process for obtaining athin layer made of a first material on a substrate made of a secondmaterial called the final substrate, including the following steps:

-   -   bonding a thick layer of a first material on one of its main        faces on the final substrate at an interface,    -   implantation of gaseous species in the thick layer of first        material to create a weakened zone delimiting said thin layer        between the interface and the weakened zone,    -   deposit a layer of third material called the self-supporting        layer on the free face of the thick layer made of first        material,    -   fracture within the structure composed of the final substrate,        the thick layer of first material and the layer of third        material, at the weakened zone to supply the substrate        supporting said thin layer.

The result is a layer that is very thin in comparison with the orders ofmagnitudes of layers conventionally transferred using the Smart Cut®process, without a problem of bubbles at the interface and with goodthickness uniformity.

Gaseous species may be implanted in the thick layer of first material byone or several implantations of identical or different gaseous specieschosen from among species for example such as hydrogen or helium.

The thick layer of first material may be composed of one or severalmaterials. It may be a layer delimited in an initial substrate during agaseous species implantation step in order to create a weakened zone inthe initial substrate, a fracture step between the thick layer of firstmaterial and the remainder of the initial substrate being made after thestep to bond the thick layer of first material on the final substrate.

The implantation of gaseous species in the initial substrate may be animplantation of hydrogen ions.

According to a first embodiment, the step to implant gaseous species inthe thick layer of first material is done after the fracture between thethick layer of first material and the remainder of the initialsubstrate.

According to a second embodiment, the step to implant gaseous species inthe thick layer of first material is done before the step to bond thethick layer of first material onto the final substrate. In general,implantations are done such that the first fracture (in the initialsubstrate) does not hinder the second fracture (within the thick layer). For example, if the fracture steps are done by heat treatment, thesteps to implant gaseous species are done under conditions such that thefracture between the thick layer of first material and the remainder ofthe initial substrate is obtained at a temperature less than thefracture temperature of said structure.

Advantageously, the self-supporting layer is fixed on the thick layer offirst material by deposition of said third material on the thick layerof first material.

The thick layer of first material may be bonded onto the final substrateby molecular bonding.

According to one variant embodiment, part of the self-supporting layeris deposited and the gaseous species are implanted in the thick layer offirst material after this partial deposit.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood and other advantages andfeatures will become clear after reading the following description givenas a non-limitative example accompanied by the attached drawings amongwhich:

FIGS. 1A to 1F are cross-sectional views illustrating a first embodimentof the process according to the invention,

FIGS. 2A to 2F are cross-sectional views illustrating a secondembodiment of the process according to the invention,

FIG. 3 is an explanatory diagram.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

FIGS. 1A to 1F illustrate a first embodiment of the process according tothe invention to obtain a thin layer of silicon on a support. Obviously,the described technique may be applied to materials other than siliconfor example such as SiC, germanium, III-V and IV-IV materials, nitrides(such as GaN) or other crystalline materials, these materials being usedalone or in combination.

FIG. 1A shows an initial substrate made of silicon 10 comprising anoxide layer 19 on the surface, typically about 0.05 μm thick, in whichone of the main faces, the oxidised face 11 is subjected to uniformioning bombardment in order to create a weakened zone 12 at a determineddistance from the face 11. The implantation is done using acceleratedhigh energy hydrogen ions (for example 210 keV) so that the createdweakened zone 12 is fairly deep from the bombarded face 11. Thus, alayer 13 with a thickness of about 1.9 μm is delimited between the face11 and the weakened zone 12, the remainder of the initial substratebeing marked with reference 14. The layer 13 may be called the thicklayer. The dose of implanted ions is chosen according to the Smart Cut®process to subsequently obtain a fracture at the weakened zone, forexample by heat treatment. The heat treatment may be assisted orreplaced by a mechanical treatment. We will use the term heat treatmentalone in the remainder of this description for simplification reasons.

FIG. 1B shows fixation of the face 11 of the initial substrate 10 on aface 21 of the final substrate 20. For example, fixation is obtained bymolecular bonding.

The structure obtained is then subjected to heat treatment at atemperature of about 480° C. This heat treatment causes a fracture ofthe structure at the weakened zone. After removal of the remainder 14 ofthe initial substrate, the result is the stacked structure shown in FIG.1C including the final substrate 20 to which the 1.9 μm thick layer 13is bonded. The thick layer 13 has a free face 15.

The structure may also be subjected to a heat treatment to reinforce itsbonding interface. For example, such a heat treatment will be done atabout 1100° C. for about 2 hours.

A surface treatment may be applied to the face 15 (by CMP, hydrogenannealing, etc.) in order to eliminate roughness. For example, if CMP isused to reduce the thickness by the order of 50 nm, a good uniformity inthe thickness of the thick layer can be maintained.

One variant could consist of depositing or thermally generating a thinlayer of oxide, for example of the order of 0.2 μm thick.

A second ionic implantation is then made, for example by hydrogen ions.This is shown in FIG. 1D. For example, the implantation energy used maybe 185 keV and the ion dose is chosen to subsequently obtain a fractureat the weakened zone thus obtained, for example by heat treatment. Theweakened zone 16 is at a depth of about 1.5 μm from the face 15. Itseparates the thick layer 13 into two sub-layers 17 and 18, thesub-layer 17 forming the required thin layer.

The next step is to deposit a layer 1 called the self-supporting layeron the face 15, as shown in FIG. 1E. It may be a layer of silicon oxide,4 μm thick, deposited by PECVD.

If a thin layer of oxide was deposited or generated before the secondimplantation, this layer will be completed here.

A heat treatment can then be applied to obtain the fracture, for examplean isothermal annealing at 600° C. This is shown in FIG. 1F. Thestructure is separated into a first part composed of a self-supporteddual layer, comprising the self-supporting layer 1 and the sub-layer 18,and a second part composed of the final substrate 20 to which the thinlayer 17 is bonded through the oxide layer 19. The dual layer could bereusable.

The final substrate 20 and the thin layer 17 can then be subjected to acleaning step, conventional steps to thin and stabilise the thin layer,illustrated for example in document FR-A-2 777 115, in order and in thecurrent optimum combination. The thin silicon layer may then beapproximately 100 nm thick.

The final substrate used may have various natures. It may be made of asemi-conducting material or an insulating material, or it may becomposed of a stack (for example a silicon substrate covered by a layerof silicon oxide).

FIGS. 2A to 2F illustrate a second embodiment of the process accordingto the invention to obtain a thin silicon layer on a support.

FIG. 2A shows an initial silicon substrate 30 for which one of the mainfaces, face 31, is subjected to uniform ionic bombardment in order tocreate a weakened zone 32 at a determined distance from the face 31.This face could also be provided with an oxide layer, for example a fewnanometers thick. As for the first embodiment of the invention, theimplantation may be done by hydrogen ions with an energy of 210 keV. Theimplantation delimits a thick layer 33 with a thickness of close to 1.9μm between the face 31 and the weakened zone 32. The remainder of theinitial substrate is marked with reference 34.

The next step shown in FIG. 2B consists of making a second ionicimplantation through the face 31. This second implantation is not asdeep as the first and the dose is lower. The implantation energy may beof the order of 50 keV. It is used to create a weakened zone 36 insidethe thick layer 33. The weakened zone 36 delimits a thin layer 37 fromthe face 31. The remainder of the thick layer 33 or the sub-layer ismarked as reference 38.

FIG. 2C shows fixation of the face 31 of the initial substrate 30 onto aface 41 of the final substrate 40 comprising an oxide layer 42 on thesurface, typically 0.05 μm thick. Fixation may be obtained by molecularbonding.

The structure obtained may then be heat treated at a relatively lowtemperature, for example 430° C., to obtain a fracture at the firstweakened zone, in other words zone 32. Implantation conditions of thetwo weakened zones were selected so as to not generate a fracture, oreven exfoliation, in the second weakened zone. The advantage of havingdone the second implantation before the thick layer is transferred isthat as a result, this second implantation is not as deep and is donethrough a normally good quality surface (better than the quality of aface obtained by fracture). Therefore, the result is a thinner weakenedzone, and therefore with a lower roughness after final fracture. Thestructure obtained is shown in FIG. 2D.

At this stage of the process, the surface treatment step may beeliminated since a self-supporting layer can be deposited directly.However, a minimum surface treatment may be done to eliminate all orpart of the roughness. It may be done by CMP, or annealing for exampleunder hydrogen or any other compatible atmosphere known to those skilledin the art, wet chemical etching or ionic etching. The surface treatmentenables removal of a few nm to a few tens of nm, thus maintaining gooduniformity of thickness. For a self-supporting layer made of SiO₂, thisminimum surface treatment enables an only slightly rough buried Si—SiO₂interface.

The next step is to make a deposit of a layer 2 called a self-supportinglayer on the thick layer 33, as shown in FIG. 2E. As mentioned above, itmay be a 4 μm thick silicon oxide layer deposited by PECVD.

A heat treatment can then be applied, for example isothermal annealingat 600° C., to obtain the fracture as shown in FIG. 2F. The structure isseparated into a first part composed of a self-supported dual layercomprising the self-supporting layer 2 and the sub-layer 38, and asecond part comprising the final substrate 40 to which the thin layer 37is bonded by means of the oxide layer 42. The dual layer could bereusable.

As described above, the cleaning and finishing steps may be performed onthe resulting stacked structure.

These two embodiments suggest that some steps may be combined and/orinverted. For example, all or part of the self-supporting layer can bedeposited and the second implantation can be done after this deposit. Inthis case, the implantation energy is corrected to take account of it.

The self-supported layer may be a silicon oxide or it may be made ofother materials, for example such as Si₃N₄, SiO_(x), Si_(x)N_(y),Si_(x)N_(Y)O_(z), Al₂O₃, SiC, sapphire, diamond, etc.

The thickness of the self-supported layer may be selected by experiment.In the case of a self-supported SiO₂ layer deposited on a silicon thicklayer, the following experiment was done to evaluate the effect of thedeposited oxide thickness on the annealing temperature, thicknessnecessary to obtain the fracture of the self-supported silicon layer.The implantation conditions were implantation energy 76 keV,implantation dose 6×10¹⁶ H⁺ions/cm² through a 400 nm thick SiO₂protective film.

FIG. 3 is a diagram in which the ordinate represents the thickness e ofthe SiO₂ deposit and the abscissa represents the annealing temperatureT. The curve shown in this diagram delimits the area in which theself-supported silicon layer is transferred (the area located above thecurve) from the area in which a “blister” occurs on the silicon layer(the zone located below the curve).

This diagram shows that the temperature of separation (or fracture) withtransfer of a self-supported dual layer does depend on the depositedoxide thickness. The temperature is higher if the oxide is thinner.Consequently, the thickness of the fractured silicon layer needs to beadded to this oxide thickness. Therefore, in particular it is possibleto deduce the minimum thickness of oxide layer necessary for thefracture to be induced at a certain temperature. Therefore, it can beseen that the “threshold” fracture thickness at 600° C. is exceeded for4 μm of deposited oxide.

Therefore, it is possible to control the thinning procedure bycontrolling the thickness of the deposited self-supporting layer, thuspreventing “blistering” and exfoliation phenomena that would occur ifthe deposited layer is thinner than the “threshold” thickness.

1. A process for obtaining a thin layer made of a first material on asubstrate made of a second material called a final substrate, theprocess comprising, in the order as hereinafter set forth: bonding athick layer of a first material by one of its main faces on the finalsubstrate at an interface followed by implanting gaseous species in thethick layer of the first material to create a weakened zone delimitingsaid thin layer between the interface and the weakened zone, orimplanting gaseous species in a thick layer of a first material tocreate a weakened zone followed by bonding said thick layer of saidfirst material by one of its main faces on the final substrate at aninterface thereby delimiting said thin layer between the interface andthe weakened zone; depositing a layer of a third material to form aself-supporting layer on a free face of the thick layer made of thefirst material; and fracturing the structure comprising the finalsubstrate, the thick layer of the first material and the layer of thethird material at the weakened zone to supply the substrate supportingsaid thin layer.
 2. The process according to claim 1, wherein implantinggaseous species further comprises implanting one or more identical ordifferent gaseous species.
 3. The process according to claim 2, whereinsaid gaseous species are selected from the group consisting of hydrogenand helium.
 4. The process according to claim 2, wherein said gaseousspecies are selected from the group consisting of hydrogen and helium.5. The process according to claim 1, wherein the thick layer of thefirst material is bonded onto the final substrate by molecular bonding.6. The process according to claim 1, wherein a part of theself-supporting layer is deposited, and the gaseous species areimplanted in the thick layer of the first material after the partialdeposit.
 7. The process according to claim 1, wherein said thin layerhas a thickness less than 0.1 μm.
 8. The process according to claim 1,comprising, in the order as hereinafter set forth: bonding a thick layerof a first material by one of its main faces on the final substrate atan interface followed by implanting gaseous species in the thick layerof the first material to create a weakened zone delimiting said thinlayer between the interface and the weakened zone; depositing a layer ofa third material to form a self-supporting layer on a free face of thethick layer made of the first material; and fracturing the structurecomprising the final substrate, the thick layer of the first materialand the layer of the third material at the weakened zone to supply thesubstrate supporting said thin layer.
 9. The process according to claim8, wherein implanting gaseous species further comprises implanting oneor more identical or different gaseous species.
 10. The processaccording to claim 9, wherein said gaseous species are selected from thegroup consisting of hydrogen and helium.
 11. The process according toclaim 1, comprising, in the order as hereinafter set forth: implantinggaseous species in a thick layer of a first material to create aweakened zone followed by bonding said thick layer of said firstmaterial by one of its main faces on the final substrate at an interfacethereby delimiting said thin layer between the interface and theweakened zone; depositing a layer of a third material to form aself-supporting layer on a free face of the thick layer made of thefirst material; and fracturing the structure comprising the finalsubstrate, the thick layer of the first material and the layer of thethird material at the weakened zone to supply the substrate supportingsaid thin layer.
 12. The process according to claim 11, whereinimplanting gaseous species further comprises implanting one or moreidentical or different gaseous species.
 13. The process according toclaim 1, consisting of, in the order as hereinafter set forth: bonding athick layer of a first material by one of its main faces on the finalsubstrate at an interface followed by implanting gaseous species in thethick layer of the first material to create a weakened zone delimitingsaid thin layer between the interface and the weakened zone; depositinga layer of a third material to form a self-supporting layer on a freeface of the thick layer made of the first material; and fracturing thestructure comprising the final substrate, the thick layer of the firstmaterial and the layer of the third material at the weakened zone tosupply the substrate supporting said thin layer.
 14. The processaccording to claim 1, consisting of, in the order as hereinafter setforth: implanting gaseous species in a thick layer of a first materialto create a weakened zone followed by bonding said thick layer of saidfirst material by one of its main faces on the final substrate at aninterface thereby delimiting said thin layer between the interface andthe weakened zone; depositing a layer of a third material to form aself-supporting layer on a free face of the thick layer made of thefirst material; and fracturing the structure comprising the finalsubstrate, the thick layer of the first material and the layer of thethird material at the weakened zone to supply the substrate supportingsaid thin layer.
 15. A process for obtaining a thin layer made of afirst material on a substrate made of a second material called a finalsubstrate, the process comprising: bonding a thick layer of a firstmaterial by one of its main faces on the final substrate at aninterface; followed by implanting gaseous species in the thick layer ofthe first material to create a weakened zone delimiting said thin layerbetween the interface and the weakened zone; followed by depositing alayer of a third material called a self-supporting layer on a free faceof the thick layer made of the first material; and fracturing within thestructure composed of the final substrate, the thick layer of the firstmaterial and the layer of the third material, at the weakened zone tosupply the substrate supporting said thin layer, wherein the thick layerof the first material is a layer delimited in an initial substrate whenimplanting a gaseous species to create a weakened zone in the initialsubstrate, and further comprising fracturing between the thick layer ofthe first material and a remainder of the initial substrate, which isperformed after bonding the thick layer of the first material onto thefinal substrate.
 16. The process according to claim 15, whereinimplanting gaseous species in the initial substrate further comprisesimplanting hydrogen ions.
 17. The process according to claim 15, whereinimplanting gaseous species in the thick layer of the first material isperformed after fracturing between the thick layer of the first materialand a remainder of the initial substrate.
 18. The process according toclaim 15, wherein implanting gaseous species in the thick layer of firstmaterial is performed before bonding the thick layer of the firstmaterial on the final substrate.
 19. The process according to claim 18,wherein fracturing is performed by a heat treatment, wherein implantinggaseous species is performed under conditions so that the fracturingbetween the thick layer of the first material and a remainder of theinitial substrate is obtained at a temperature less than the fracturetemperature of said structure.
 20. The process according to claim 15,wherein said thin layer has a thickness less than 0.1 μm.